Plasmin promotes human dendritic cell endocytosis of dead cancer cells and T cell activation

Presentation of antigen from necrotic cancer cells within tumour microenvironments to T cells requires efficient uptake by DCs. However, intact necrotic tumour cells present physical barriers to direct endocytosis. Here, we investigate the non-canonical role of the serine protease plasmin and its regulatory network in tumour cell endocytosis by human monocyte-derived dendritic cells (mDCs) and downstream T cell stimulation. Pre-treating heat-killed melanoma cells with plasmin significantly enhanced their endocytosis by mDCs, an effect reversed by the physiological inhibitor α2-antiplasmin. Live-cell imaging and flow cytometry demonstrated that mDCs predominantly internalise micron-scale cellular debris rather than whole cells, and that plasmin activity "digests" dead-cell particles, thereby enabling uptake of particles of optimal diameter (∼1-3 μm). Enhanced endocytosis directly translated to increased CD4+ T cell activation. Furthermore, both heat-killed melanoma cells and mDCs functioned as lysine-dependent plasminogen-binding surfaces capable of generating localised proteolytic activity. Plasmin treatment modestly reduced DC cell-surface PD-L1 expression and altered specific inflammatory cytokines. These findings reframe plasmin as a modulator of immune-tumour interactions and highlight important methodological considerations regarding serum-derived proteases in experimental immunology.